The present invention relates to instruments for detecting small particles, and more particularly to devices that utilize light scattering in monitoring particle flux.
The need to control particulate contamination arises in a variety of environments, including hospital operating rooms, in the pharmaceutical industry and in aerospace manufacturing. The need arises in processing semiconductor wafers, where contamination by fine particles can substantially reduce yield.
In semiconductor chip fabrication, one well known method of monitoring particle contamination is to periodically insert test wafers into the process equipment, after which the test wafers are observed with a surface defect scanner or microscope, and the particles counted. This technique is labor intensive and does not provide continuous, real time monitoring of particulate contamination.
Another approach, disclosed in U.S. Pat. No. 4,783,599 (Borden), employs a flow of fluid through a pipe section or other volume defining structure, with a laser beam directed through a window into and across the pipe section, perpendicular to fluid flow through the section. The beam is focused near the center of the pipe section, and particles passing through the beam near the focal point scatter light. The scattered light is focused through a lens onto a detector, while light in the beam itself is blocked and thereby prevented from reaching the lens. The detected scattered light indicates the passage of a particle through the beam, and also can indicate the size of the particle. A problem with this arrangement is that the focal spot of the laser is small, generally less than a cubic millimeter. Thus, only a small fraction of the particles in fluid flow are detected.
The Borden patent further discloses a particle detector in which a collimated light beam is directed through a pipe section perpendicular to the direction of liquid flow through the pipe section, and into a beam stop tube. A pair of photodiodes, mounted to a window along the pipe section and located on opposite sides of the beam, respond to scattered light in a manner that detects particles but avoids mistaken identification of bubbles in the liquid as particles.
U.S. Pat. No. 4,825,094 (Borden et al) discloses a device for monitoring particle fallout in real time, including a laser source for generating a beam and a pair of spaced apart mirrors for reflecting the laser beam multiple times between the mirrors, then to a beam stop. A cylindrical tube, attached to the housing that mounts the laser and mirrors, surrounds an opening in the housing proximate the beam, to ensure that particle containing air is relatively still and virtually static near the reflected beam. The multiple reflections of the beam form a sheet or grid of light, whereby some particles in the flow intersect the sheet and scatter light.
In U.S. patent application Ser. No. 07/416,958 filed Oct. 4, 1989 and assigned to the assignee of the present application, a single particle detector employs opposed cylindrical mirrors and a flat mirror to define a sensing volume in which a laser beam is reflected multiple times between the mirrors to form a sheet of light. Single particles intersecting the sheet are measured based on light extinction. Light scattering, however, is preferred in connection with smaller particles, which may be insufficient in profile or in concentration to generate a reading based on extinction.
A light scattering photometer is disclosed in U.S. Pat. No. 4,178,103 (Wallace), in which liquid is formed into a curtain for particle detection. A laser beam is focused to provide a beam waist at the curtain. Direct beam energy is reflected away from the beam path by a radiant energy trap, while scattered light within a predetermined range of angles is admitted through a mask to a focusing lens, focused at a field stop wheel, diverged to another focusing lens and then focused at a detector.
While these detectors perform satisfactorily in their intended environments, there remains a need for a device that effectively monitors particle flux in a wide variety of manufacturing process environments.
Therefore, it is an object of the present invention to provide a particle sensing device usable in situ to continuously monitor particle flux in connection with a flow of fluid including a particulate suspension.
Another object of the invention is to provide a particle flux counting device employing light scattering, in which the viewing volume is substantially enlarged beyond a focal point or beam waist, without the need for multiple reflections of the beam.
A further object is to provide, in a particle monitoring device utilizing light scattering, an effective means for blocking the unscattered beam to prevent its reaching the sensing means employed to detect scattered light.
Yet another object is to provide a light scattering particle sensing device that is relatively inexpensive, yet effective in monitoring particle flux in fluids, over a wide range of fluid flow velocities.